This application claims the priority of Germany Application No. 197 51 977.6, filed Nov. 25, 1997, the disclosure of which is (are) expressly incorporated by reference herein.
The invention relates to a control system for the supercharging pressure of a supercharged internal combustion engine whose exhaust gas turbocharger has an actuator for the supercharging pressure, and has, in particular an adjustable turbine geometry.
It is already known from European Patent document EP 0 747 585 A2 to fit the control system with an open-loop control device as well as, in addition, a regulating device. The supercharging pressure is subjected to open-loop control in the lower load range of the internal combustion engine. The loop controller is only cut-in beginning in the upper load range. The supercharging pressure is then subjected to closed-loop control. The open-loop control device remains active in this case even in the load range subjected to closed-loop control, in order to ensure stable operation of the turbo charger.
For the purpose of setting the supercharging pressure, the open-loop control device reads out from characteristic diagrams desired values which are stored electronically as a function of the motor speed and the fuel injection quantity. The desired values are converted into a specific pulse duty factor with which the actuator is driven. For the purpose of subjecting the supercharging pressure to closed-loop control, the regulating device determines desired values, prescribed by a desired value/actual value comparison and determined actual values of the supercharging pressure, to determine a regulation pulse duty factor which is fed to the operating path to the actuator. The regulation pulse duty factor is added to the control pulse duty factor.
A switch is provided for the purpose of cutting in the closed-loop control during the transition from the lower to the upper load range. The switch is fed a fuel injection quantity signal and a speed signal on the input side. Cutting in is not performed until the fuel injection quantity signal overshoots a speed-dependent threshold which is read out from a characteristic curve. The regulation is cut out (discontinued) if, upon falling, the fuel quantity signal undershoots the speed-dependent threshold which, for the purpose of achieving a hysteresis effect, is read out form a lower characteristic curve running at a distance from and approximately parallel to the upper characteristic curve. A precontrol effect is provided in closed-loop control by: (1) the open-loop control device, likewise cut in, and (2) the looping through, associated therewith, of the control desired value of the supercharging pressure.
Exhaust gas turbochargers with adjustable turbine geometry have a high dynamic response in the supercharging pressure buildup. In the case of operating the variable turbine geometry (VTG) turbocharger with the known supercharging pressure control system, the supercharging pressure repeatedly overshoots the desired value when there is a rapid supercharging pressure build up. This leads directly to instances of detrimental overshooting of the exhaust gas counterpressure in the exhaust section of the internal combustion engine. A high exhaust gas counterpressure in consequence of the stagnation effect of the supercharger turbine acts on the drive pistons of the internal combustion engine and demands a higher level of work from the pistons to eject the exhaust gases. Since the overshooting of the exhaust gas counterpressure must be avoided at all costs, the known supercharging pressure control system cannot be used to achieve a build up of supercharging pressure which is optimum in terms of torque, particularly in non-stationary operating states of the supercharged internal combustion engine.
It is the object of the invention to develop the control system for the supercharging pressure such that the dynamic properties of the exhaust gas turbocharger with adjustable turbine geometry with respect to the build up of supercharging pressure are used in all operating states of the internal combustion engine, and that it is possible to build up the supercharging pressure in a way which is optimum in terms of torque.
This object is achieved according to the invention by a control system which permits both open-loop control of the supercharging pressure with desired values which are prescribed for the open-loop control device in the operating path to the actuator as a function of the engine speed and of the current fuel injection quantity, and closed-loop control of the supercharging pressure which always ensures a supercharging pressure which is optimum in terms of operating point and a build up of supercharging pressure which is optimum in terms of torque. The supercharging pressure is advantageously subjected to closed-loop control in further operating ranges of the internal combustion engine, in particular in the case of an increasingly higher partial load and in full load operation. In the case of a low partial load, the regulating device is switched off and the supercharging pressure is subjected to open-loop control. This is regarded as advantageous because of the fact that the characteristic curve of the exhaust gas turbocharger is flat in this load range. The changeover state is prescribed for the changeover device arranged between the regulating device and the operating path to the actuator. The changeover device uses a fuel injection parameter which is a function at least of speed, in order to indicate the changeover state between closed-loop operation and open-loop operation.
The control pulse duty factor as the current output signal of the open-loop control device, and the regulation pulse duty factor, additively fed in the closed-loop operation, of the regulating device proportionately form a pulse duty factor of the control system for influencing the actuator. A precontrol pulse duty factor is added before the actuator is reached. The precontrol pulse duty factor is calculated by a separate precontrol device from a required change in the desired value of an operating parameter of the internal combustion engine, and is not fed to the operating path to the actuator until downstream of the adder for the regulation pulse duty factor. The system deviation of the supercharging pressure as a result of the desired value/actual value comparison is preferably fed to the precontrol device as an input signal for determining the precontrol pulse duty factor. The proportion of the graduated precontrol pulse duty factor in the actuation pulse duty factor finally arriving at the actuator rises with the gradient of the required change in the desired value, as a result of which a dynamic build up of supercharging pressure is achieved, in particular in non-stationary operating states with the strengthened influence of the precontrol device on the adjustable turbine geometry.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.